In response to the above questions I will first say that ALL writing should be considered as potentially flawed. Writers always mis-remember things, exaggerate, make stuff up, or otherwise make errors. The fact that Luke says something about an event that Mark does not is most likely because Luke added something that did not actually occur.

Consider the supposed prophesy about being “crucified and on the third day be raised again”. To have such knowledge Jesus must have been divine. On the other hand, for some follower of Jesus (Luke or someone else that Luke got the story from) to CLAIM that Jesus said this to his followers is a very easy thing to do. And since Luke’s agenda in his gospel is to show the divinity of Jesus, this story about the prophesy would help push that agenda. As long as people believe the story, Luke’s agenda gets pushed along just fine.

So, answer to the first question, Mark did not have the women speak like Luke did because the “remember” phrase was never spoken, and Mark had never heard the made-up story about it being spoken. Luke wanted Jesus to seem divine so he said that Jesus prophesied the “third day” event. He needed to tell that story so he added all those words about the prediction.